A railway sleeper is one of the railway accessories that plays a vital role in railway construction. In the rail structure, the railway sleeper is used to bear the train load, transverse and longitudinal horizontal forces transmitted from the rail, and transmit them evenly to the track bed or steel beam; meanwhile, the railway sleeper is also used to maintain the rail direction, gauge, position, etc. Therefore, the railway sleeper is required to have sufficient strength, rigidity and durability, and the size of the railway sleeper is required to be convenient for the fixation of the rail, having the ability to resist longitudinal and transverse displacement of the rail track.
At present, railway sleepers mainly include reinforced concrete railway sleepers and wooden sleepers. The mass of reinforced concrete railway sleeper is great. When prefabricated in the factory, the reinforced concrete railway sleeper needs to be embedded with plastic sleeves, and it is impossible to achieve on-site drilling for installation. Due to the requirements of high precision for the pre-embedded plastic sleeves, it is difficult to achieve close cooperation between the fastening systems and the rails. Especially in the turnout area, the error of pre-embedded plastic sleeve in the railway sleeper is prone to waste product, manpower and material resources. Moreover, the rigidity of the reinforced concrete railway sleeper is too large. The reinforced concrete railway sleeper does not have the shock absorption effect, and harms such as cracks, etc. are even prone to occur after long-term operation of the train, threatening driving safety and requiring regular maintenance and replacement, thereby increasing management costs. The production of wooden sleepers requires a large number of high-quality hardwoods. The wooden sleepers are impregnated with creosote to improve their corrosion resistance, but the use of the chemical preservative to treat the wooden sleepers has negative effects on the environment and the health of workers. Wooden sleepers are prone to suffer from corrosion in the natural environment such as sun, rain, etc., thus cracks, flaws and holes occur. During the operation, the fastening rail spikes are easy to loosen and need to be maintained and repaired regularly, and after about 5 to 10 years, they must be replaced, which greatly increases the management cost of the railway sleepers. On one hand, the supply of high-quality wood for manufacturing railway sleepers is declining, on the other hand, as the volume of transportation, the time and labor costs is increasing, the cost of maintenance and replacement of wooden sleepers is increasing.
With the improvement of people's environmental awareness, the control of construction cost, and the requirements for fast, safe and comfortable travel, composite railway sleepers will gradually replace the wooden sleepers, and will even be widely used in the ballast area.
At present, research on composite railway sleepers has begun in China and abroad. For example, the recycled waste plastics are used to make railway sleepers in United States, waste tires are used as the main material to make railway sleepers in South Korea, research on railway sleepers are mainly made of sawdust, and ongoing in Russia, research on hollow glass fiber reinforced plastics (FRP) railway sleepers are ongoing in India and China, and composite railway sleepers are made of polyurethane foam and fiberglass in Japanese and China. Although performance, such as the corrosion resistance, etc., of these composite railway sleepers are superior to those of wooden sleepers, the comprehensive properties such as strength, fatigue resistance, anti-pulling strength of the rail spike, gripping ability of the rail spike and anti-shearing property cannot meet the requirements of China Railway for composite railway sleepers. For example, the composite railway sleepers made of recycled waste plastics meet the requirements on the strength, but brittle fracture occurs in the fatigue resistance test. The hollow FRP railway sleepers meet the requirements on the strength, but anti-pulling strength of the rail spike thereof is less than 3 t, far less than the technical requirements of the anti-pulling strength of the rail spike greater than 6 t, and cracks occur in the fatigue resistance test. Serious collapses occur in the fiber-reinforced polyurethane composite railway sleepers produced in Japan and China during the fatigue resistance test, thus the fiber-reinforced polyurethane composite railway sleepers are not suitable for use in ordinary railways and heavy loading railways in China, and are only suitable for use in light rails and subways at home and abroad. The above composite railway sleepers cannot achieve all the performance requirements of the railway sleepers.
The polyurethanes used in the fiber-reinforced rigid polyurethane foam composite railway sleepers from Japan and China are formed by reacting polyether polyol with isocyanate. The used polyether polyol has high functionality. The reaction time and curing rate of the polyurethane are fast, and the curing is almost completed in about 3 minutes. Moreover, the selected polyether polyol has high viscosity and poor fluidity. Thus, the problem of uniform impregnation of polyurethane and fiber before the polyurethane is cured is difficult to be solved. Therefore, the fiber-reinforced rigid polyurethane foam composite railway sleepers currently produced at home and abroad have low fiber content, uniform impregnation of polyurethane and fiber, high local polyurethane content, large foam pores, high local fiber content, and drying yarn phenomenon, thus resulting in low and uniform strength, and unstable quality. During the production process, the resin foam expansion produces a large amount of casting fin due to low fiber content and high resin content thereof, which not only causes waste of the product, increases the production cost, but also causes great damage to the equipment and the mold. Moreover, because the problem of impregnation of polyurethane and reinforcing fibers cannot be solved, it is impossible to produce fiber-reinforced rigid polyurethane foam composite railway sleepers with relatively high fiber content, and even more impossible to produce fiber-reinforced rigid polyurethane foam composite railway sleepers with relatively high fiber content and high fiber density. At present, the fiber-reinforced rigid polyurethane foam composite railway sleepers having a density higher than 840 kg/m3 and a fiber content higher than 60% have not been reported and available on the market.